As a conventional water heating system, there is disclosed a water heating system including a water-refrigerant heat exchanger that performs heat exchange between refrigerant and tank water by using a heat pump as a heat source, a hot water/cold water supply circuit that returns the tank water boiled by the heat exchange in the water-refrigerant heat exchanger to a tank and stores the tank water therein, and control means that performs a boiling operation by operating a pump provided in the hot water/cold water supply circuit, feeding the tank water to the water-refrigerant heat exchanger by the pump, and boiling the tank water by the heat exchange with the refrigerant in the water-refrigerant heat exchanger.
Water such as tap water and ground water normally contains hardness components such as calcium and magnesium. In the water heating system as described above, a temperature of calcium or magnesium contained in tap water or the like is increased in a heating section for tank water of the water-refrigerant heat exchanger. When the temperature exceeds a degree at which the calcium or magnesium has solubility in water, the calcium or magnesium is transformed into calcium carbonate or the like (referred to as scale below), and precipitates on a surface or the like of the water-refrigerant heat exchanger. The scale causes problems that heat exchange efficiency of the water-refrigerant heat exchanger is reduced, and a flow path is closed.
Thus, to solve the above problems, there has been proposed means for preventing scale deposition when the heat pump is stopped by stopping a compressor of the heat pump after the boiling operation is stopped, while continuing the operation of the pump to circulate the water and decrease an outlet temperature of a water-refrigerant heat exchange section to the same level as an inlet temperature (for example, see Patent Literature 1).
A water heating system described in Patent Literature 1 is a water heating system that directly exchanges heat between refrigerant and tap water, and especially in Europe, a water heating system is generally employed that circulates water heated by refrigerant and then exchanges heat between the circulated water and tap water (for example, see Patent Literature 2).